Composition and method for weldable tin-free steel having a chromium bilayer

ABSTRACT

A method for producing a tin free steel having double layers consisting of a lower layer of flatly deposited metallic chromium and an upper layer of insoluble hydrated chromium oxide which is characterized by a dissolution of soluble hydrated chromium oxide after the formation of three layers consisting of a bottom layer of metallic chromium, a middle layer of insoluble hydrated chromium oxide and an upper layer of soluble hydrated chromium oxide on a steel base by using a chromic acid electrolyte with a small amount of fluoride compound. 
     By using this tin free steel, a welded can body can be produced at high speed without the removal of the plated layer in the welded part because this tin free steel has an excellent weldability.

This application is a continuation of application Ser. No. 859,033,filed May 30, 1989, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a method for producing a tin free steelhaving excellent weldability. In detail, the invention relates to amethod for producing a tin free steel having double layers consisting ofa lower layer of flatly deposited metallic chromium and an upper layerof an uniformly formed insoluble hydrated chromium oxide. It ischaracterized by a dissolution of soluble hydrated chromium oxide afterthe formation of three layers consisting of a bottom layer of metallicchromium, a middle layer of insoluble hydrated chromium oxide and anupper layer of soluble hydrated chromium oxide on a steel base by usinga chromic acid electrolyte with a small amount of fluoride compound.

By using this tin free steel, a welded can body can be produced at highspeed without the removal of the plated layer in the welded part.

SUMMARY

The method for producing a tin free steel having excellent weldabilityaccording to the present invention was developed by the detailedinvestigation.

The objective of the present invention can be accomplished by providinga tin free steel having double layers consisting of a lower layer ofmetallic chromium of the restricted amount and an upper layer ofinsoluble hydrated chromium oxide of the restricted amount on a steelbase by using a chromic acid electrolyte with a small amount of fluoridecompound (fluoride bath). The method according to the present inventionis characterized by the following factors:

(1) The use of a fluoride bath for the formation of said three layers ona steel base.

(2) The dissolution of soluble hydrated chromium oxide being the upperlayer of said three layers after the formation of said three layers on asteel base by an immersion into said fluoride bath.

(3) The restriction of the range in the amount of the deposited metallicchromium.

(4) The restriction of the range in the amount of the formed insolublehydrated chromium oxide.

The tin free steel according to the present invention is easily weldedat high speed without the removal of the plated layer and also can beused in applications wherein excellent weldability is required, such asfood can bodies, aerosol can bodies and miscellaneous can bodies whichare lacquered except the welded part before welding. Furthermore, thetin free steel according to the present invention can be also used forcan ends and drawn cans because it has excellent corrosion resistanceafter lacquering comparable to that of the ordinary tin free steel.

DESCRIPTION Background and Objective

In the description, soluble hydrated chromium oxide means the hydratedchromium oxide which is easily dissolved before drying into the fluoridebath or chromic acid solution without additives before drying andinsoluble hydrated chromium oxide means the hydrated chromium oxidewhich is difficult to dissolve into the fluoride bath or chromic acidsolution without additives even before drying.

Recently, the change from expensive electrotinplates to cheaper tin freesteel having double layers consisting of a lower layer of metallicchromium and an upper layer of hydrated chromium oxide has rapidly takenplace in the field of food and beverage cans, five gallon cans andmiscellaneous cans. This is because the tin used for the production oftinplate is expensive and tin free steel has excellent lacquer adhesioncompared with that of tinplate.

An ordinary metal can made of tin free steel consists of two can endsand a single can body, except for drawn can. In the case of tin freesteel, the seaming of the can body is generally carried out with nylonadhesives by using the Toyo Seam (Trade name) and Mira Seam (Trade name)method. Another method of seaming a tin free steel can body by electricwelding is also well known. In the case of the seaming of a tin freesteel can body by electric welding such as the Soudronic process,however, the metallic chromium layer and the hydrated chromium oxidelayer must be mechanically or chemically removed from the tin free steelsurface in order to easily obtain a well seamed can body at high speed.Therefore the corrosion resistance in the welded part of the tin freesteel can body becomes remarkably poor, even if this welded part iscoated with lacquer after welding.

From the background described above, the development of a can materialwhich is cheaper than tinplate and is easily weldable at high speedwithout the removal of the plated layer, has been required, especiallyin the field of food cans.

Recently, various methods for producing tin free steel which can beeasily welded at high speed without the removal of the plated layer havebeen proposed. For instance, the methods shown in Japanese PatentPublication Nos. Sho 57-19752, Sho 57-36986 and Laid-Open JapanesePatent Application Nos. Sho 61-213398, Sho 63-186894 have been alreadyknown.

Japanese Patent Publication No. Sho 57-19752 relates to a tin free steelwith excellent weldability having double layers consisting of a lowerlayer of metallic chromium of 3 to 40 mg/m² and an upper layer ofnon-metallic chromium which is mainly chromium oxide of 2 to 15 mg/m² aschromium. This Sho 57-19752 intends to improve the weldability of tinfree steel by the formation of a porous metallic chromium layer with asmall amount of metallic chromium. However, it is considered that notonly the weldability but also corrosion resistance are poor, because theiron oxide film having high electric resistance is formed by theoxidation of the steel base deliberately exposed through the pore ofmetallic chromium layer during the lacquer curing.

Japanese Patent Publication No. Sho 57-36986 is characterized by the useof a chromic acid electrolyte with a small amount of anions such assulfate ion, nitrate ion and chloride ion in order to produce a tin freesteel with excellent weldability, formability and lacquerability whichhas metallic chromium of 0.5 to 30 mg/m² and hydrated chromium oxide of2 to 50 mg/m² as chromium. This method for producing the tin free steelintends to improve the corrosion resistance, which becomes poor by adecrease in the amount of metallic chromium, by the improvement of thequality of the hydrated chromium oxide layer. However, the weldabilityof the tin free steel obtained by this method will be not improvedbecause the exposed steel surface is oxidized by heating the lacquercoated tin free steel similar to Sho 57-19752 described above.

Laid-Open Japanese Patent Application No. Sho 61-213398 relates to a tinfree steel for a welded can with excellent corrosion resistance afterlacquering which has flatly deposited metallic chromium of 10 to 40mg/m² and uniformly formed hydrated chromium oxide of 3 to 30 mg/m² aschromium. The method for producing this tin free steel is characterizedby the dissolution of a part of the deposited metallic chromium by ananodic treatment after the formation of a double layer consisting ofmetallic chromium and hydrated chromium oxide. However, it is consideredthat the weldability of the tin free steel obtained by this method willbe not improved because of the increase of the exposed steel base withthe increase of the pore in the metallic chromium layer.

Laid-Open Japanese Patent Application No. Sho 63-18689 relates to a tinfree steel for a welded can having metallic chromium of 50 to 150 mg/m²and hydrated chromium oxide of 5 to 20 mg/m² as chromium. The tin freesteel obtained by Sho 63-186894 is characterized by the granulardeposition of metallic chromium. This tin free steel was developed basedon the fact that an electric contact resistance, which is used for oneof the index of the evaluation of the weldability, became lower by thegranular deposition of metallic chromium.

The weldability is usually evaluated by an available secondary currentrange in welding, that is, the wider the secondary current range inwelding, the better the weldability. On determining the availablesecondary current range in welding, the upper limit corresponds to thewelding conditions in which some defect such as splashing is found andthe lower limit corresponds to the welding conditions in which thebreakage occurs in the welded part by the tearing test. However, theweldability is usually evaluated by a simple method of electric contactresistance measurement, which has an apparent correlation to theavailable secondary current range in welding, because a large number ofsamples are necessary in order to determine the available secondarycurrent range in welding.

Namely, the lower the electric contact resistance, the wider theavailable secondary current range in welding.

Although the electric contact resistance has an apparent correlation tothe available secondary current range in welding for tin free steelshaving almost the same morphology of metallic chromium, this correlationis not recognized for tin free steels having the different morphology ofmetallic chromium. For example, in the case of a tin free steel havinggranularly deposited metallic chromium obtained by Sho 63-186894, theavailable secondary current range in welding is narrow in spite of thelow electric contact resistance.

Therefore, the weldability of tin free steel is not evaluated by onlythe electric contact resistance, although the electric contactresistance is used as the index of the evaluation of the weldability fortin free steels having almost the same morphology of metallic chromium.

In general, there are two well-known types of method for producing anordinary tin free steel. The first type is a one-step method in whichmetallic chromium and hydrated chromium oxide are formed in oneoperation by using one electrolyte composition. The second type is atwo-step method in which metallic chromium is formed first by using oneelectrolyte composition as a chromium plating solution, and thenhydrated chromium oxide is formed on the metallic chromium layer byusing another electrolyte composition. In both types of processes, achromic acid electrolyte with a fluoride compound (fluoride bath), witha sulfate compound (sulfate bath) or with both additives (mixed bath)are usually used. It has been known in the report by K. Yoshida et al.(Kinzoku Hyomen Gijutsu, vol. 30, No. 7, 1979, page 338) that the filmin the ordinary tin free steel produced by the methods described aboveis constructed of three layers consisting of a bottom layer of metallicchromium, a middle layer of insoluble hydrated chromium oxide which ismainly equivalent to chromium oxide and an upper layer of solublehydrated chromium oxide. Namely, the hydrated chromium oxide isconstructed by two layers having different structures.

DESCRIPTION OF THE DRAWING

FIG. 1 shows the relationship between the amount of metallic chromiumand the passivation current of the steel base exposed through themetallic chromium layer after the dissolution of the formed hydratedchromium oxide by an immersion into hot sodium hydroxide solution. Ashows that obtained by using a fluoride bath (CrO₃ : 100 g/L, NH₄ F:5g/1) under the cathodic current density of 50 A/dm² at 50° C. B and Cshow those obtained by using a mixed bath (CrO₃ :150 g/1, H₂ SO₄ :0.8g/1, Na₂ SiF₆ :5 g/1) and a sulfate bath (CrO₃ :160 g/1, H₂ SO₄ :2.5g/1) under the cathodic current density of 50 A/dm² at 60° C.respectively.

FIG. 2 shows one example of the effect of the immersion time into achromic acid electrolyte on the amount of the remained insolublehydrated chromium oxide. A and B show those obtained by using thefluoride bath and the mixed bath described above for the formation andthe dissolution of hydrated chromium oxide, respectively.

FIG. 3 shows the effect of the amount of metallic chromium on theelectric contact resistance of the tin free steel produced using thefluoride bath described above before heating (B) and after heating at210° C. for 20 minutes (A).

FIG. 4 shows the effect of the amount of metallic chromium on theavailable secondary current range in welding of the tin free steelproduced by using the fluoride bath described above after heating at210° C.for 20 minutes.

The black points in FIG. 3 and FIG. 4 show the electric contactresistance and the available secondary current range of the tin freesteel having granular deposition of metallic chromium, which is producedby an anodic treatment after cathodic treatment of a steelsheet in amixed bath described above, after heating at 210° C. for 20 minutes,respectively.

DETAILED DESCRIPTION OF THE INVENTION

The steel base used for the production of the tin free steel accordingto the present invention can be any cold rolled steel sheet customarilyused in manufacturing electrotinplate and ordinary tin free steel.Preferably, the thickness of the steel base is from about 0.1 to about0.35 mm.

The tin free steel according to the present invention is produced by thefollowing processes: degreasing with an alkali and pickling with anacid, then water rinsing, then forming said three layers consisting ofmetallic chromium, insoluble hydrated chromium oxide and solublehydrated chromium oxide, then dissolving of the soluble hydratedchromium oxide by an immersion into the electrolyte, and water rinsingand then drying.

In the process described above, water rinsing after the formation ofsaid three layers may be added, if the chromic acid solutions used forthe formation of said three layers and the dissolution of solublehydrated chromium oxide have the different composition in order tocontrol the concentration of each composition.

In the present invention, it is preferable to employ a fluoride bath forthe following reasons:

(1) The area f the steel base exposed through the pore of the formedmetallic chromium layer is smaller than that in a sulfate bath and mixedbath at the same amount of metallic chromium as shown in FIG. 1. Namely,the steel base is uniformly covered with a small amount of metallicchromium compared with that in a sulfate bath. In the case of a mixedbath, the addition of a large amount of sulfuric ion is not preferablebecause the area of the steel base exposed from the metallic chromiumlayer increases.

(2) The flat deposition of metallic chromium is obtained by using afluoride bath. On the other hand, the granular deposition of metallicchromium is obtained by using a sulfate bath. Therefore the steelsurface is sufficiently covered with a small amount of metallic chromiumcompared with that in a sulfate bath.

(3) The thin and uniform soluble hydrated chromium oxide is formed byusing a fluoride bath, since the structure of soluble hydrated chromiumoxide is not disturbed by the fluorine incorporated into the hydratedchromium oxide which has nearly the same volume as the hydroxyl radicalor bonded water in the hydrated chromium oxide. On the other hand, thethick and non-uniform soluble hydrated chromium oxide is formed by usinga sulfate bath, since the structure of the soluble hydrated chromiumoxide is disturbed by the incorporated sulfate ion having the samevolume as trivalent chromium coordinated by a hydroxyl radical or bondedwater with a coordination number of 6. The formation of a thinnersoluble hydrated chromium oxide is preferable in order to easilydissolve the formed soluble hydrated chromium oxide in a short time.

(4) The film having a thinner and uniform insoluble hydrated chromiumoxide is formed by using a fluoride bath compared with that in a sulfatebath.

It is indispensable in the present invention that each layer of theformed three layers is thin and uniform in order to produce continuouslyat high speed a tin free steel having excellent weldability. Therefore,the use of a fluoride bath in the present invention is preferablebecause the fluoride bath has the various merits described above.

In the case of the fluoride bath, the following electrolytic conditionsfor the formation of said three layers on a steel base should beselected:

Concentration of chromic acid: 50 to 300 g/l, more preferably 80 to 200g/l.

Concentration of fluoride compound: 1.0 to 10.0 weight %, morepreferably 1.0 to 8.0 weight % of chromic acid.

Temperature of the electrolyte: 40° to 60° C. more preferably 50° to 60°C.

Cathodic current density: 20 to 100 A/dm², more preferably 40 to 100A/dm².

The amount of soluble hydrated chromium oxide in the formed three layersdecreases with an increase of the chromic acid concentration with asuitable weight ratio of fluoride compound under higher cathodic currentdensity at higher temperature of the electrolyte.

Therefore, it is not preferable to use a fluoride bath having below 30g/l of chromic acid under the cathodic current density below 20 A/dm² atthe temperature of the fluoride bath below 40° C. in order to form thinsoluble hydrated chromium oxide. Furthermore, the use of a fluoride bathhaving below 30 g/l of chromic acid, the temperature of the fluoridebath of above 60° C. and the current density of below 20 A/dm² is notpreferable, because the current efficiency for the deposition ofmetallic chromium decreases remarkably. However, the concentration ofchromic acid above 300 g/l and the cathodic current density above 100A/dm² is not suitable for the deposition of a small amount of metallicchromium from an economical point of view.

The presence of fluoride compound in the electrolyte used for producingthe tin free steel according to the present invention is indispensablefor a uniform three layers. If the weight % of fluoride compound tochromic acid is below 1.0 or above 10.0, the current efficiency for thedeposition of metallic chromium remarkably decreases, besides a decreaseof the uniformity of the deposited metallic chromium layer.Particularly, at below 1.0 weight % of fluoride compound to chromicacid, the weldability becomes remarkably poor, because the thick andnon-uniform insoluble hydrated chromium oxide is formed.

It is preferable that the fluoride compound is at least one compoundselected from the group consisting of hydrofluoric acid, fluoboric acid,fluosilicic acid, ammonium bifluoride, an alkali metal bifluoride,ammonium fluoride, an alkali metal fluoride, ammonium fluoborate, analkali metal fluoborate, ammonium fluosilicate, an alkali metalfluosilicate and aluminum fluoride.

The formed soluble hydrated chromium oxide which is the upper layer ofsaid three layers should be removed before water rinsing and drying inthe process for producing the tin free steel having excellentweldability according to the present invention. It is suitable to employthe fluoride bath, which is used for the formation of said three layerson a steel base, in order to dissolve the soluble hydrated chromiumoxide in said three layers from an economical point of view, although achromic acid solution without additives or with another additives can bealso used. The concentration of chromic acid and the temperature of thesolution for the dissolution of the formed soluble hydrated chromiumoxide should be controlled above 30 g/l and above 40° C. respectively,in order to sufficiently dissolved the soluble hydrated chromium oxidein a short time. The concentration of chromic acid above 300 g/l and thetemperature above 60° C.are not suitable for the dissolution of thesoluble hydrated chromium oxide from an economical point of view. Theimmersion time is a very important factor in the process for producingthe tin free steel according to the present invention. As shown in FIG.2, the amount of insoluble hydrated chromium oxide becomes almostconstant in above 2.5 seconds of immersion time. In the presentinvention, the immersion time should be controlled in the range of 2.5to 10 seconds, because the immersion above 10 seconds needs a largespace in a case of the continuous production of the tin free steelaccording to the present invention at high speed. An anodic treatment inthe fluoride bath after the formation of said three layers is alsoconsidered for the dissolution of the formed soluble hydrated chromiumoxide. However, this method is not suitable in the present invention,because the area of the steel base exposed through the metallic chromiumlayer increases by the anodic dissolution of metallic chromium.

The amount of metallic chromium in the tin free steel according to thepresent invention should be controlled in the range of 45 to 90 mg/m².If the amount of metallic chromium is below 45 mg/m², excellentweldability after heating for lacquer curing is not obtained, becausethe area of the steel base exposed through the formed metallic chromiumlayer increases remarkably as shown in FIG. 1. The increase in the areaof the steel base exposed through the formed metallic chromium leads tothe increase in the electric contact resistance as shown in FIG. 3 andthe narrow range of the secondary current in welding as shown in FIG. 4,because the iron oxide film having higher electric resistance is formedby the oxidation of the exposed steel base. If the amount of metallicchromium is above 90 mg/m², the weldability becomes also poor by anincrease in the amount of metallic chromium having bad forging abilitycompared with that in the steel base as shown in FIG. 3 and FIG. 4,although the area of the steel base exposed from the metallic chromiumlayer decreases with an increase of the amount of metallic chromium. Itis found from FIG. 3 and FIG. 4 that the electric contact resistance hasan apparent correlation to the available secondary current range inwelding in the case of the tin free steel having almost the samemorphology of metallic chromium according to the present invention.

On the contrary, other tin free steel having a granular deposition ofmetallic chromium has a narrow available secondary current range inwelding, in spite of low electric contact resistance as shown in FIG. 3and FIG. 4.

It is generally well known that the weldability of the ordinary tin freesteel depends on the amount of hydrated chromium oxide having higherelectric resistance after heating for curing the coated lacquer. Thatis, the lower the amount of hydrated chromium oxide, the better theweldability. However, the decrease in the amount of hydrated chromiumoxide is not suitable from the point of the corrosion resistance afterlacquering, although the weldability is improved. Especially, thepresence of soluble hydrated chromium oxide is not suitable forproducing a tin free steel having excellent weldability, because theuniformity of the formed soluble hydrated chromium oxide is poor, evenif the amount of soluble hydrated chromium oxide is small. Practically,it is impossible to uniformly weld the can body of the ordinary tin freesteel having a small amount of soluble hydrated chromium oxide withoutthe removal of the plated layer at high speed, because the electrolyticcontact resistance is different locally in the welded part.

On the other hand, the insoluble hydrated chromium oxide in said threelayers is uniformly formed in comparison with the soluble hydratedchromium oxide. Furthermore, the amount of the formed insoluble hydratedchromium oxide is almost constant without the effects of theelectrolytic conditions for the formation of said three layers. In thepresent invention, it is preferably to control in the range of 3 to 7mg/m² as chromium the amount of the formed insoluble hydrated chromiumoxide. It is industrially difficult to form the insoluble hydratedchromium oxide having below 2 mg/m² and above 10 mg/m² as chromium withmetallic chromium and soluble hydrated chromium oxide except somespecial electrolytic conditions such as the use of a chromic acidelectrolyte with little amount of additives. If the amount of insolublehydrated chromium oxide is below 3 mg/m², the corrosion resistance afterlacquering becomes remarkably poor, although the weldability isimproved. The weldability becomes gradually poor with an increase in theamount of insoluble hydrated chromium oxide same as the increase in theamount of soluble hydrated chromium oxide. In the present invention, theallowable upper limit in the amount of insoluble hydrated chromium oxideis 7 mg/m² as chromium in order to uniformly weld without the removal ofthe plated layer at high speed.

The present invention is illustrated by the following examples. Theseexamples serve to illustrate the invention and not to limit it. Otherswill be obvious to those skilled in the art.

In Example 1 to Example 5, a cold rolled steel sheet having a thicknessof 0.22 mm was treated by the following process after electrolyticallydegreasing in a solution of 70 g/l of sodium hydroxide, water rinsingand then pickling in a solution of 100 g/l of sulfuric acid, followed byrinsing with water.

Formation of three layers consisting of a bottom layer of metallicchromium and a middle layer of insoluble hydrated chromium oxide and anupper layer of soluble hydrated chromium oxide by using a fluoride bath,then water rinsing, followed by immersion into a fluoride bath (chromicacid solution in Example 4 and Example 5) followed by water rinsing andthen drying.

In each Example, the conditions are shown in detail. Condition A showsthe electrolytic conditions for the formation of said three layersconsisting of a bottom layer of metallic chromium, a middle layer ofinsoluble hydrated chromium oxide and an upper layer of soluble hydratedchromium oxide. Condition B shows the conditions for the dissolution ofthe formed soluble hydrated chromium oxide.

EXAMPLE 1

    ______________________________________                                        Condition A                                                                   Composition of electrolyte                                                    CrO.sub.3          200 g/l                                                    NH.sub.4 F          7 g/l                                                     Temperature of electrolyte                                                                        50° C.                                             Cathodic current density                                                                          50 A/dm.sup.2                                             Condition B                                                                   Solution for dissolution                                                                         the same composition as                                                       the above electrolyte                                      Temperature of solution                                                                           50° C.                                             Immersion time      3 seconds                                                 ______________________________________                                    

EXAMPLE 2

    ______________________________________                                        Condition A                                                                   Composition of electrolyte                                                    CrO.sub.3          150 g/l                                                    NaF                 5 g/l                                                     Temperature of electrolyte                                                                        58° C.                                             Cathodic current density                                                                          70 A/dm.sup.2                                             Condition B                                                                   Solution for dissolution                                                                         the same composition as                                                       the above electrolyte                                      Temperature of solution                                                                           58° C.                                             Immersion time      5 seconds                                                 ______________________________________                                    

EXAMPLE 3

    ______________________________________                                        Condition A                                                                   Composition of electrolyte                                                    CrO.sub.3          80 g/l                                                     Na.sub.2 SiF.sub.6  6 g/l                                                     Temperature of electrolyte                                                                       55° C.                                              Cathodic current density                                                                         30 A/dm.sup.2                                              Condition B                                                                   Solution for dissolution                                                                         the same composition as                                                       the above electrolyte                                      Temperature of solution                                                                          55° C.                                              Immersion time      6 seconds                                                 ______________________________________                                    

EXAMPLE 4

    ______________________________________                                        Condition A                                                                   Composition of electrolyte                                                    CrO.sub.3             100 g/l                                                 HBF.sub.4              3 g/l                                                  NaF                    1 g/l                                                  Temperature of electrolyte                                                                           45° C.                                          Cathodic current density                                                                             50 A/dm.sup.2                                          Condition B                                                                   Solution for dissolution                                                                            CrO.sub.3 100 g/l                                       Temperature of solution                                                                              50° C.                                          Immersion time         8 seconds                                              ______________________________________                                    

EXAMPLE 5

    ______________________________________                                        Condition A                                                                   Composition of electrolyte                                                    CrO.sub.3             100 g/l                                                 NH.sub.4 F             5 g/l                                                  Temperature of electrolyte                                                                           45° C.                                          Cathodic current density                                                                             50 A/dm.sup.2                                          Condition B                                                                   Solution for dissolution                                                                            CrO.sub.3 100 g/l                                       Temperature of solution                                                                              45° C.                                          Immersion time         8 seconds                                              ______________________________________                                    

COMPARATIVE EXAMPLE 1

The same kind of steel sheet pretreated as in Example 1 was plated withmetallic chromium and then was post treated under the followingconditions, followed by rinsing with water and drying.

    ______________________________________                                        Condition for metallic chromium deposition                                    Composition of electrolyte                                                    CrO.sub.3              200 g/l                                                H.sub.2 SO.sub.4        1.5 g/l                                               Temperature of electrolyte                                                                            50° C.                                         Cathodic current density                                                                              50 A/dm.sup.2                                         Condition for post-treatment                                                  Composition of electrolyte                                                    CrO.sub.3               60 g/l                                                Temperature of electrolyte                                                                            55° C.                                         Cathodic current density                                                                              10 A/dm.sup.2                                         ______________________________________                                    

COMPARATIVE EXAMPLE 2

The same kind of steel pretreated as in Example 1 was treated under thefollowing conditions and then rinsed with water and dried.

    ______________________________________                                        Condition for treatment                                                       ______________________________________                                        Composition of electrolyte                                                    CrO.sub.3              50 g/l                                                 Na.sub.2 SiF.sub.6      1.5 g/l                                               Temperature of electrolyte                                                                           55° C.                                          Cathodic current density                                                                             20 A/dm.sup.2                                          ______________________________________                                    

COMPARATIVE EXAMPLE 3

The same kind of steel sheet pretreated as in Example 1 was plated withmetallic chromium and then anodically treated under the followingconditions.

    ______________________________________                                        Condition for metallic chromium deposition                                    Composition of electrolyte                                                    CrO.sub.3           50 g/l                                                    Na.sub.2 SiF.sub.6   6 g/l                                                    H.sub.2 SO.sub.4     0.8 g/l                                                  Temperature of electrolyte                                                                        55° C.                                             Cathodic current density                                                                          50 A/dm.sup.2                                             Condition for anodic treatment                                                Composition of electrolyte                                                                        the same composition as                                                       the above electrolyte                                     Temperature of electrolyte                                                                        55° C.                                             Anodic current density                                                                             3 A/dm.sup.2                                             Time for electrolysis                                                                              0.2 seconds                                              ______________________________________                                    

After that, the thus treated steel was cathodically treated under thefollowing condition, followed by rinsing and drying.

    ______________________________________                                        Condition for cathodic treatment                                              ______________________________________                                        Composition of electrolyte                                                    CrO.sub.3              60 g/l                                                 Temperature of electrolyte                                                                           55° C.                                          Cathodic current density                                                                             15 A/dm.sup.2                                          ______________________________________                                    

COMPARATIVE EXAMPLE 4

The same kind of steel sheet pretreated as in Example 1 was plated withmetallic chromium and then was anodically treated under the followingconditions.

    ______________________________________                                        Condition for metallic chromium deposition                                    Composition of electrolyte                                                    CrO.sub.3           200 g/l                                                   Na.sub.2 SiF.sub.6   7.5 g/l                                                  H.sub.2 SO.sub.4     0.5 g/l                                                  Temperature of electrolyte                                                                         50° C.                                            Cathodic current density                                                                           60 A/dm.sup.2                                            Condition for anodic treatment                                                Composition of electrolyte                                                                        the same composition as                                                       the above electrolyte                                     Temperature of electrolyte                                                                         60° C.                                            Anodic current density                                                                             5 A/dm.sup.2                                             Time for electrolysis                                                                              0.2 seconds                                              ______________________________________                                    

After that, this treated steel sheet was cathodically treated under thefollowing conditions, followed by rinsing with water and drying.

    ______________________________________                                        Condition for cathodic treatment                                              ______________________________________                                        Composition of electrolyte                                                    CrO.sub.3              50 g/l                                                 NaF                     4 g/l                                                 Temperature of electrolyte                                                                           40° C.                                          Cathodic current density                                                                             15 A/dm.sup.2                                          ______________________________________                                    

COMPARATIVE EXAMPLE 5

The same kind of steel sheet pretreated as in Example 1 was cathodicallytreated under the same conditions as in Example 2. After that, thistreated steel sheet was rinsed with water and dried without thedissolution of the formed soluble hydrated chromium oxide.

Comparative Example 1, 2 and 3 and 4 shows one example in JapanesePatent Publication Nos. Sho 57-19752, Sho 57-36986, Laid-Open JapanesePatent Application Nos. Sho 61-213398 and Sho 63-186894 respectively.

The area of steel exposed through the formed metallic chromium layer,the electric contact resistance, the available range of secondarycurrent in welding and the corrosion resistance after lacquering of thethus treated steel sheet in the above described Examples and Comparativeexamples were evaluated by the following testing methods after themeasurement of the amounts of metallic chromium and chromium ininsoluble hydrated chromium oxide by the fluorescent X-ray method.

The results are shown in the attached Table.

(1) Area of steel base exposed from the formed metallic chromium layer(Uniformity of metallic chromium layer).

The hydrated chromium oxide of the sample which was cut to a size of 50mm×50 mm was dissolved by an immersion into 300 g/l of sodium chloridesolution for 5 minutes at 95° C. After that, the sample was sealed withvinyl tape except for the tested area having a diameter of 30 mm. Thecurrent for the passivation of the steel base in the sealed sample wasmeasured by using an anodic polarization method at 125 mV/min of apolarization speed. This current value corresponds to the area of thesteel base exposed through the metallic chromium layer, namely, showsthe uniformity of the metallic chromium layer.

(2) Electric contact resistance

At first, the sample was cut to a size of 20 mm×100 mm after heating at210° C . for 20 minutes, and then a pair of samples were inserted intobetween a pair of a copper disk electrodes (diameter:65mm, thickness:2mm) rotating at 5 m/min. The electric contact resistance was calculatedfrom the voltage between a pair of the copper disk electrodes wherein 5amperes of direct current was employed and 50 kg of load was added.

(3) Available range of secondary current in welding

The sample was cut to a size of 100 mm×50 mm after heating at 210° C.for 20 minutes. A pair of sample which was overlapped with a width of0.4 mm was welded under 50 kg of load, 60 Hz of frequency at 7.2 m/min.The upper limit of secondary current was determined by the conditions inwhich some defect such as splashing was found and the lower limit of onewas determined by the conditions in which the breakage occurred in thewelded part by tearing test by using a large number of samples.Available secondary current range in welding was determined from thedifference of the secondary current described above.

(4) Corrosion resistance after lacquering (Test 1)

The sample was baked at 210° C.for 10 minutes after coating with 60mg/dm² of an epoxy-phenolic type of lacquer. The coated sample wasimmersed into the solution containing 1.5% of citric acid and 1.5% ofsodium chloride for 2 weeks at 38° C., after the surface of the coatedsample was cross-hatched with 10 μm of width and 15 μm of depth by arazor.

The corrosion in the scratched part of the coated sample was dividedinto 5 ranks, namely, 5 was excellent, 4 was good, 3 was fair, 2 waspoor and 1 was bad.

(5) Filiform corrosion resistance after lacquering (Test 2)

The sample which was cross-hatched after lacquer coating in (4) wasformed by an erichsen testing machine. The degree of rust in thescratched part of the formed sample which was immersed into a solutioncontaining 3% of sodium chloride was divided into 5 ranks, namely 5 wasexcellent, 4 was good, 3 was fair, 2 was poor and 1 was bad, after thestorage for 10 days under 85% of relative humidity at 45° C.

                                      TABLE 1                                     __________________________________________________________________________                 Area of steel                                                                         Electric                                                                           Available                                                  Amount of                                                                           exposed from                                                                          contact                                                                            secondary                                                                           Corr. resistance                                     Cr.sup.o                                                                         Cr.sup.ox                                                                        Cr.sup.o layer                                                                        resistance                                                                         current                                                                             after lacquering                                     (mg/m.sup.2)                                                                        (mA/30 mmΦ)                                                                       (mΩ)                                                                         range (A)                                                                           Test 1                                                                            Test 2                                    __________________________________________________________________________    Example 1                                                                            50 5  44      15   150   5   4                                         Example 2                                                                            85 3  18      18   150   5   4                                         Example 3                                                                            55 6  34      20   125   5   5                                         Example 4                                                                            80 4  32      18   125   5   4                                         Example 5                                                                            60 3  34      20   150   5   4                                         Comp. Ex. 1                                                                          18 10 800     80    25   5   4                                         Comp. Ex. 2                                                                           4 5  1000    65    25   3   2                                         Comp. Ex. 3                                                                          35 7  440     38    50   5   3                                         Comp. Ex. 4                                                                          95 15 120      2    50   5   3                                         Comp. Ex. 5                                                                          85 7  20      36    75   5   4                                         __________________________________________________________________________     Remarks                                                                       *1 Cr.sup.o shows metallic chromium and Cr.sup.ox shows Cr in hydrated        chromium oxide                                                           

What is claimed:
 1. A method for producing a tin free steel sheet forwelding at high speed having thereon essentially a double layer whereinsaid double layer consists of a lower layer of flatly deposited metallicchromium and an upper layer of insoluble hydrated chromium oxide whereinsaid upper layer is insoluble in the aqueous chromic acid solution fromwhich the layers are deposited which consists of:(1) forming threelayers on said steel sheet consisting of a bottom layer of metallicchromium in the amount of 45-90 mg/m², a middle layer in the amount of3-7 mg/m² hydrated chromium oxide as chromium insoluble in said chromicacid solution and an upper layer of hydrated chromium oxide soluble insaid chromic acid solution by cathodic treatment at a cathodic currentdensity of 20 to 100 A/dm² of a substantially clean steel sheet in asulfate-free aqueous chromic acid solution consisting of water, 50 to300 g/l chromic acid and one or more fluorine compounds in an amount offrom 1.0 to 10.0 weight percent of the chromic acid at a temperature of40°-60° C.; (2) dissolving said soluble hydrated chromium oxide by animmersion of the steel sheet covered with said three layers formed bystep (1) into the aqueous chromic acid solution in the absence of anapplied electric current for an immersion time of 2.5-10 sec.; and (3)rinsing with water and drying.
 2. The method of claim 1 wherein saidfluoride compound is at least one compound selected from the groupconsisting of hydrofluoric acid, fluoboric acid, fluosilicic acid,ammonium bifluoride, an alkali metal bifluoride, ammonium fluoride, analkali metal fluoride, ammonium fluoborate, an alkali metal fluoborate,ammonium fluosilicate, an alkali metal fluosilicate and aluminumfluoride.
 3. A tin-free steel sheet having essentially a chromiumbilayer as produced by the method of claims 1 or 2.